The present invention relates to the general field of undersea fluid transport pipes that rest on the sea bottom or that provide a bottom-to-surface connection for transporting hydrocarbons, e.g. oil and gas, from undersea production wells.
Undersea fluid transport pipes are commonly used in offshore hydrocarbon production. Thus, in an offshore production field, a plurality of wells are generally worked, which wells may be spaced apart from one another by several kilometers, or indeed tens of kilometers. The fluids coming from the various wells need to be collected by pipes that are laid on the sea bottom and transferred by bottom-to-surface connection pipes from an undersea pipe resting on the sea bottom to a surface installation that receives them, e.g. on board a ship or at a collection point situated onshore.
There exist various types of undersea pipes that are used for transporting fluids. The invention relates more particularly to coaxial pipes of the pipe in pipe (PIP) type, in which an inner steel tube transports the fluids and an outer steel tube coaxial with the internal tube, and also referred to as the “outer shell”, is in contact with the surrounding medium, i.e. with the water.
In general, such coaxial pipes are assembled on land from unit lengths (referred to as double, triple, or quadruple “joints”, with the term “quad-joints” being used herein for quadruple sections of tube), which unit lengths present a length lying in the range 10 meters (m) to 100 m, depending on the load-holding capacity of the laying system. Such quad-joints are then taken to sea on a laying ship.
During laying, the quad-joints are connected to one another on board the ship progressively as they are being laid at sea. This laying may be performed by using a J-lay tower positioned on the laying ship. With J-laying, the undersea pipe is typically lowered from the laying ship while it is practically vertical (at an angle in the range +30° to −10° relative to the vertical). J-laying is single-catenary laying in which the almost vertical angle of inclination of the pipe decreases progressively as it moves downwards until it takes on the slope of the sea bottom.
Given their specificity, undersea fluid transport pipes are designed to achieve a high level of thermal performance, and specific versions have been developed to be better adapted to great depths, i.e. to withstand pressure at the bottom of the sea. Specifically, since the pressure of water is substantially 0.1 megapascals (MPa), i.e. about 1 bar, for a depth of 10 m, the pressure that undersea pipes need to be able to withstand is then about 10 MPa, i.e. about 100 bar, for a depth of 1000 m, and about 30 MPa, i.e. about 300 bar, for a depth of 3000 m.
Furthermore, undersea pipes are subjected to high levels of mechanical stress, both while they are being laid on the bottom of the sea via a J-lay tower, which gives rise to high levels of deformation (in particular in bending) in each of the quad-joints of the pipe, and also during the production stage (internal thermal stresses and stresses due to external forces). Specifically, once the undersea pipe has been installed on the bottom of the sea and the network is in production, the inner tubes of the quad-joints of the pipe are subjected to the high pressure of the fluids they are transporting (which pressure can exceed 100 bar). Furthermore, it often happens that the sea bottom on which the pipe is resting moves, which gives rise to movements of the pipe, and thus to stresses in them.
However, the mechanical stresses to which undersea pipes are subjected, and more particularly the inner tubes of PIP pipes, and also the temperatures to which they are subjected (hydrocarbons coming from undersea production wells are at a temperature of about 70° C.), run the risk of leading to major damage or even breakage of the pipes.